The main rotor blades of many helicopters and other rotorcraft are fabricated from composite materials due to the superior stiffness and strength properties and corrosion resistance of composites. Such high stiffness and strength properties provide an increased fatigue life for the rotor blades in the high-vibration environment of a helicopter. In addition, composite materials provide a means for tailoring the mass and stiffness characteristics at different locations along the span of a rotor blade to optimize the aeroelastic performance of the rotor blade.
In this regard, a main rotor blade may be constructed with different types of materials positioned at different locations within the airfoil shape of the rotor blade to achieve specific structural stiffness and balance characteristics. Different materials may also be positioned at specific locations along the airfoil shape or material thicknesses may be varied along the length to provide operational durability for the rotor blade. For example, a metallic skin may be included on the leading edge of a composite spar of a rotor blade to provide erosion durability for the rotor blade.
The use of different types of materials for different components within the rotor blade may result in imbalances in the thermal expansion characteristics of the dissimilar materials. For example, the metallic skin may have a coefficient of thermal expansion that is higher that the coefficient of thermal expansion of the composite spar. The metallic skin may be adhesively bonded to the composite spar at an elevated cure temperature inside a cure tool. The differing coefficients of thermal expansion of the metallic skin and composite spar may result in the metallic skin shrinking along a lengthwise direction to a greater extent than the shrinkage of the composite spar. Because of cross-linking that occurs during adhesive cure, a rigid bondline is formed between the metallic skin and composite spar. The rigid bondline results in stress buildup between the metallic skin and the composite spar upon cool down from the cure temperature which may result in shape distortion such as bowing in the cured spar assembly.
Conventional approaches for minimizing shape distortion during the manufacturing of rotor blades include the use of cure tools that are designed to be highly rigid and/or which have a low coefficient of thermal expansion to minimize distortion during the cure cycle in an attempt to maintain the rotor blade in a desired (e.g., straight) shape. Complex holding features may also be incorporated into sub-assembly parts and subsequent cure tools in an attempt to lock the rotor blade components into a desired straight condition. Unfortunately, conventional approaches fail to adequately address the shape distortion (e.g., bowing) that occurs in a composite rotor blade as a result of the imbalance in the dissimilar materials with regard to thermal contraction after cure. Such shape distortion in cured composite subassemblies may present challenges in fitting the cured subassemblies into subsequent cure tools and compromise the integrity of the final part.
As can be seen, there exists a need in the art for a system and method for minimizing or eliminating shape distortion in cured composite articles comprised of dissimilar materials.